Electric heaters for livestock watering tanks have been of two general types. One type has been the submerged heater that rests on the bottom of the watering tank. The other type has been the floating water heater.
Typical of the submerged type is that which is described by McKinstry in U.S. Pat. No. 4,068,116. In this patent, McKinstry discloses a unit in which plates are placed above and below the heating element to protect it from accidental damage, and in which a metal band senses the temperature of the heating element and transfers the heat to a thermostat.
Typical of the floating type is the unit which is described by Landgraf in U.S. Pat. No. 2,561,932. Landgraf suspends the heating element beneath a float.
Also typical of the floating type are the units which are disclosed by Brodie in U.S. Pat. No. 2,430,272, Rietz in U.S. Pat. No. 2,454,091, and Temple in U.S. Pat. No. 2,472,178.
All of the above designs, both submerged and floating, have one weakness in common: none of them accurately sense the temperature of the heating element.
It is important to accurately sense the temperature of the heating element because, otherwise, if the water heater is accidently thrown out of the watering tank by a drinking animal, it may become hot enough to start a fire in straw, or other combustible material, with resulting loss of property and livestock.
McKinstry recognized the need to communicate the temperature of the heating element to the thermostat; but he attempted to transfer the heat with a thin and elongated metal strip. However, using the elongated metal strip, the heat transfer not only would be relatively slow, but highly inaccurate.
Further, McKinstry did not disclose any provision for making intermetallic contact between the heating element and the metal strip, or between the metal strip and the thermostat by bonding, so there is little assurance of good heat transfer in a new unit, and no assurance of good heat transfer in a unit that has oxidized or corroded.
In addition to the above-mentioned problems of transferring heat through the metal strip of McKinstry, when the heater is submerged in water, the water would cool the metal strip before the metal strip could transfer heat from the heating element to the thermostat. So, in actuality, the thermostat of McKinstry would sense water temperature when the heater is in the water, and would be inaccurate and unreliable in sensing the temperature of the heating element if accidentally removed from the livestock watering tank.
None of the prior art inventors, as listed above, seemed to recognize the need of sensing the temperature of the heating element as a safety factor. Brodie intentionally senses ambient air temperature; and the others sense water temperature rather than heating element temperature.
That is, the thermostat of Landgraf is so remote from the element that it is apparent that he was only considering the sensing of water temperature; and, by inspection, it is apparent that the conductivity between water and the heating element, and between water and the thermostat, are both greater than the conductivity between the heating element and the thermostat in the unit which is disclosed by Temple.
In addition, because all of the prior art, as listed above, either directly exposes the heating element to the water, or else bonds the heating element to the outside of a thin metallic float, it appears that none of the prior art inventors have recognized the desirability of reducing the watt density of the heater as another means of reducing the danger of accidentally setting fires, and as a means for reducing the danger of burning the nose or mouth of a drinking animal.
Further, none of the prior art designs provide the structural ability to withstand physical abuse such as the weight of a horse or a cow. Even though McKinstry does place thin protective metal plates above and below the heating element, it is obvious that these metal plates would provide only superficial protection against accidental damage of the heating element.
Therefore, none of the prior art has disclosed, or suggested, a design in which the temperature of the heating element is closely controlled by a thermostat, or a unit in which accidental damage is prevented by adequate structural design.